It is because you have generated another earth that there is a potential for dangerous voltages (under fault status) to be generated on your house earth - that makes your 'home brew' earth potentially live.

Was - back in 1975, now I'm not even allowed to replace a socket in a kitchen, yet as part of my work, I'm connecting equipment to 3 phase, 125A - which shows how strange regs are.

If you follow the usual stuff you find on the internet, you will find that extensions to the equi-potential zone - effectively the ground area of your home, are cited as not allowed under 17th regs, and the new 18th version appears I think next month. In fact, the regs don't state this at all - but there are sizes of bond required to allow for an extra ground. The usual suggestions for sheds and outbuildings, because of the zone extension (which is NOT banned in the regs) is simply that whatever you do has to be safe, so the only sensible guidance is that if you don't understand what you are doing, then you don't do it. The regs do not set any kind of "you MUST" for outside buildings, apart from the usual disconnect times that apply to everything. Many sparks will read the regs and deduce that the best way to do a shed at the end of the garden is to use a ground local to it, and NOT connect to the EPZ in the house, and the new ground keeps isolation - but this fails to take into account the fact that you then have a floating supply in the shed and the quality of the ground depends on soil conditions and the weather. Sandy soils and a hot summer make the ground efficiency pretty poor, and many sparks would be looking at a proper earth leakage circuit breaker vs residual current device? Connecting the new ground to the house ground can, as I said, produce currents that have to be grounded out - and while the current is low in non-fault conditions, a real serious fault could put the join between the two grounds under severe stress, hence why 10mm2 or even 16mm2 cable would be sensible keeping in mind the maximum current capacity of the supply - up to 100A available any many consumer units - you need to be able to carry the current to allow the up front protection to do it's job.

It's very clear that some thought is required, but remember some TV/radio studios do exactly the same thing - a separate technical earth, that all the equipment uses for clean grounds. It can cause all kinds of problems, so it's not seen that much nowadays - somebody could bring in a piece of equipment where the chassis is connected to the mains cable ground, but then the device links general ground to the technical ground through the audio connection cable - which isn't likely to stand much over current at all. The upshot is that if extra grounding reduces a hum, this indicates the existing grounding was not that effective, and probably floating above 0V where it should have been.

To conclude. I can only speak personally, but getting a good ground to the outside is essential, and I would have no qualms in bonding a local ground to the house ground making sure the bond is sufficiently heavy. If you read 17th (and I assume 18th) nothing at all contradicts this. If you research properly the equi-potential zone, and extensions, you'll find plenty of explanation, but it's a bit heavy on theory. In fact, the really advanced electrical engineers, not the jobbing sparks, are quite scathing about the misunderstanding and publicity the extensions of grounds gets.

As with all things safety - it's common to say that if you have to ask, then you're not competent to make the decision. Here, we really don't have enough information on the exact circumstances and that's why advice is difficult. If you ever do this - joining two grounds from different locations, it's quite disconcerting to see and hear a spark when you do it. Does that mean there's a fault, or is it simply that the new earth is doing it's job? Commons sense says one thing, but physics says the other. Sinking that current can be an improving safety process - but to work out if it is, you need to be able to analyse what is actually happening.

If you MUST search on line - I suggest the IET for proper, decent advice. An organisation that is very tight on the rules and safety. Read up on types of grounding - this topic is a good start. Some good explanations of different supply variations.

He didn't say he had cured it. However - the advice is valid. If you don't know what you are doing with something that can kill you - don't even try it.

This is why electrical forums always have safety warnings, and while everyone should be using the same set of regulations, some people may have been at university for 3 years, and others on a 6 week course. Electrical theory is also severely based in maths, and I know plenty of electricians who can't do maths to save their lives. Some sparks can do motors, 3 phase and star/delta calculations. Others can't. Some are totally competent wiring a house because they do this all the time - but have no experience at all of work in say a factory or theatre (where I work). It's quite common for the cheapest quote to come from somebody totally unaware of how we would do things. They even make assumptions on technical issues. A BBC sparks, for example, will understand technical separation of earths, and house electrician won't - assuming they are the same. They aren't. I don't think we should encourage random application of earthing - because in fault conditions the current capability is quite frightening.